The present invention relates generally to methods of producing a security document or device, and in particular to security documents or device which include an optically diffractive structure, such as a diffraction grating or like device. The invention is applicable to the production of banknotes, and it will be convenient to hereinafter describe the invention in relation to that exemplary application. It is to be appreciated, however, that the invention is not to be limited to that application.
The practice of applying diffraction gratings and like optically detectable diffractive and other structures to security documents and devices, such as credit cards, bank notes and cheques, has become widely adopted. Currently, diffraction gratings and other structures are produced in multi-layer thin films supported on thin carrier structures. The structures are then transferred from the thin film carrier substrate to the security document or device, typically by using a hot-stamping process. Reflective optical diffraction devices conventionally contain an ultra-thin vacuum metallised reflective layer, usually aluminium, adjacent to the diffractive structure within the multi-layer structure.
A number of disadvantages are associated with the production and application of optically diffractive and other devices in this manner. Firstly, the complex and highly specialised process required to produce the multi-layer optically diffractive device structure, which may contain at least four and frequently more layers, is costly. Secondly, an extra process step is required to transfer the optically diffractive device from the carrier substrate onto the security document or device. Thirdly, the resistance of the optically diffractive device as a whole to physical wear and tear and chemical attack is poor due to the weakness of the vacuum metallised reflective layer.